The present invention relates to a new ultra-high-tenacity polyvinyl alcohol fiber (abbreviated as PVA fiber hereinafter) and a process for producing the same. More particularly, it relates to a PVA fiber which has incomparably better mechanical properties such as tensile strength and initial modulus than the conventional known PVA fiber, or even has ultra-high tenacity comparable to that of the aromatic polyamide fiber or aramid fiber, and to a process for producing the same.
PVA fiber is superior to polyamide fiber (nylon) and polyester fiber in mechanical properties (particularly modulus), resistance to sun light or outdoor exposure, and hydrophilic nature. Because of these characteristic properties, it finds a variety of uses in industrial applications such as fishing nets, tire cord, and cement reinforcement.
Such conventional PVA fiber is produced usually by the wet spinning process. According to this method, an aqueous solution of PVA is extruded from a spinneret into a coagulating bath such as a saturated aqueous solution of inorganic salt, in which the polymer solidifies to form filaments. The filaments then undergo washing, drawing, and drying, and finally acetalization that makes the filaments water-insoluble. In order to improve the mechanical strength of thus obtained PVA fiber, there have been proposed several methods. For example, according to Japanese Patent Publication No. 9209/1973, the polymer solution is incorporated with boric acid or a salt thereof, and according to Japanese Patent Laid-open No. 128309/1981, the wet-spun or dry-spun PVA filaments are drawn at least ten times and then heat-treated at a temperature higher than the drawing temperature under tension that keeps the filaments at a fixed length or permits the filaments to shrink up to 3%.
The PVA fiber produced by these processes is certainly improved in mechanical properties such as modulus over the conventional PVA fiber; but yet it does not attain the good mechanical properties comparable to those of aramid fiber.
The conventional process for producing PVA fiber has a disadvantage in that it requires acetalization to make the fiber water-insoluble. This step inevitably deteriorates the mechanical properties of the resulting PVA fiber.
A process for producing PVA fiber without the insolubilizing step was disclosed in Japanese Patent Publication No. 16675/1968. According to this disclosure, PVA is dissolved in dimethyl sulfoxide (abbreviated as DMSO hereinafter), and the resulting solution is extruded from a spinneret into a coagulating bath containing an organic solvent such as ethanol, methanol, benzene, and chloroform, or a mixture thereof with DMSO. The PVA fiber produced according to this process exhibits a certain degree of water-insolubility even though it does not undergo the above-mentioned insolubilizing step; nevertheless, it does not have water resistance satisfactory in practical use. Moreover, it is poor in mechanical properties. For example, its tensile strength is only about 10 g/d. Thus it is not regarded as a high-tenacity fiber comparable to aramid fiber.